The present invention relates to a method of tuning ultrasonic devices and more particularly to a method for tuning ophthalmic phacoemulsification handpieces.
A typical ultrasonic surgical device suitable for ophthalmic procedures consists of an ultrasonically driven handpiece, an attached cutting tip, an irrigating sleeve and an electronic control console. The handpiece assembly is attached to the control console by an electric cable and flexible tubings. Through the electric cable, the console varies the power level transmitted by the handpiece to the attached cutting tip and the flexible tubings supply irrigation fluid to and draw aspiration fluid from the eye through the handpiece assembly.
The operative part of the handpiece is a centrally located, hollow resonating bar or horn directly attached to a set of piezoelectric crystals. The crystals supply the required ultrasonic vibration needed to drive both the horn and the attached cutting tip during phacoemulsification and are controlled by the console. The crystal/horn assembly is suspended within the hollow body or shell of the handpiece by flexible mountings. The handpiece body terminates in a reduced diameter portion or nosecone at the body's distal end. The nosecone is externally threaded to accept the irrigation sleeve. Likewise, the horn bore is internally threaded at its distal end to receive the external threads of the cutting tip. The irrigation sleeve also has an internally threaded bore that is screwed onto the external threads of the nosecone. The cutting tip is adjusted so that the tip projects only a predetermined amount past the open end of the irrigating sleeve. Ultrasonic handpieces and cutting tips are more fully described in U.S. Pat. Nos. 3,589,363, 4,223,676, 4,246,902, 4,493,694, 4,515,583, 4,589,415, 4,609,368, 4,869,715 and 4,922,902, the entire contents of which are incorporated herein by reference.
In use, the ends of the cutting tip and irrigating sleeve are inserted into a small incision of predetermined width in the cornea, sclera, or other location. The cutting tip is ultrasonically vibrated along its longitudinal axis within the irrigating sleeve by the crystal-driven ultrasonic horn, thereby emulsifying the selected tissue in situ. The hollow bore of the cutting tip communicates with the bore in the horn that in turn communicates with the aspiration line from the handpiece to the console. A reduced pressure or vacuum source in the console draws or aspirates the emulsified tissue from the eye through the open end of the cutting tip, the cutting tip and horn bores and the aspiration line and into a collection device. The aspiration of emulsified tissue is aided by a saline flushing solution or irrigant that is injected into the surgical site through the small annular gap between the inside surface of the irrigating sleeve and the cutting tip.
The horn (transducer) assembly, including both piezoelectric and high endurance limit inert materials, used in ultrasonic handpieces must be carefully tuned for proper operation. As used herein, "tuning" is the process of finding and tracking the correct resonant frequency of the handpiece operating under loaded or unloaded conditions. Operating the handpiece at resonance takes advantage of the transducer's energy storage capabilities, which occurs only at resonance. With proper tuning, the transducer will store mechanical energy while operating unloaded and release this energy into the material being cut when loaded. As a consequence, for short periods of time, large amounts of energy can be directed into the material by the transducer itself and not by the transducer's power source. This allows the power source to be designed to handle only the steadystate power requirement of the transducer and not the loaded transients which can be many times higher.
Prior to the present invention, the usual way of determining the resonant frequency of a transducer was to compare the phase angle between the voltage applied to the transducer and the current drawn by the transducer. When alternating voltage is applied to a circuit, current will flow through the circuit. The amount of current is determined by dividing the voltage by the impedance of the circuit according to Ohm's Law. If the circuit is purely resistive, the impedance is equal to the total resistance in the circuit and the current equals the voltage divided by the circuit resistance.
When the voltage and current waveforms are viewed on an oscilloscope for a particular circuit, if the circuit is inductive, current will lag voltage and, if the circuit is capacitive, the voltage will lag the current. The time difference between the points when the voltage and current waveforms intersect the zero axis is measured in trigonometric terms by the phase angle .phi.. For purely resistive circuits, .phi.=0 and the voltage and the current are said to be in phase. For purely inductive circuits, .phi.=90.degree. and for purely capacitive circuits, .phi.=-90.degree. and the voltage and the current are said to be out of phase.
For circuits containing all three elements, resistors, inductors and capacitors, there will be some frequencies where the total impedance of the circuit will appear purely resistive even though the circuit contains reactive elements. These frequencies are the resonant frequencies. Consequently, one method of determining the resonant frequencies of a complex circuit is to apply an alternating voltage to the circuit and vary the frequency until the phase angle .phi. between the voltage and current is zero. The frequencies where this condition occurs are the resonant frequencies. As discussed above, when driving a circuit having both resistive and reactive components, it is important to know the value of the phase angle .phi. because the power absorbed by the circuit is directly proportional to the cosine of the phase angle (cos(.phi.)). For a phase angle equal to zero, cos(0)=1 (unity) and the transfer of power from the source to the circuit is at a maximum, this is the case for purely resistive loads. However, if .phi.=90.degree. or if .phi.=-90.degree., as is the case for reactive loads, the cos(.phi.)=0 so there is no power transferred through the circuit. Cos(.phi.) is referred to as the power factor.
Ultrasonic devices driven by piezoelectric or magnorestrictive elements present complex equivalent circuits that are a combination of capacitors, inductors and resistors and generally have more than one resonant frequency. In fact, for these electromechanical transducers, the resonant frequencies occur in pairs of closely spaced frequencies where the impedance is resistive and the phase angle .phi. is zero. One of these resonant frequencies is called the series resonant frequency and the other resonant frequencies is called the parallel resonant frequency or the antiresonance. When the ultrasonic device is driven at either of these frequencies the power factor is equal to unity and the transfer of power is maximized.
However, the phase angle of the ultrasonic device is dependent on the amount of loading on the transducer. This loading is generally understood to mean resistive-type loading that will have a damping effect on the vibrations of the transducer. The series and parallel resonant frequencies exist only if the transducer is unloaded or only lightly loaded. If the resistive loading on the transducer is increased above a threshold amount, the transducer will no longer resonate because the load totally dampens the vibrations of the transducer. When this condition occurs, .phi. will no longer be zero and the transfer of power will no longer be optimum. The addition of a tuning inductor in series with the transducer will produce a resonant and, consequently, a zero phase angle condition no matter how heavy the load. This inductor, however, adds a third resonant frequency to the circuit when the device is operated under a no load or small load condition. This third frequency must be differentiated from the series and parallel resonant frequencies during tuning.
Prior art tuning methods generally use phase locked looped circuits with very narrow tuning ranges centered around one of the resonant frequencies discussed above. While this approach is suitable for drivers used in combination with transducers having operating resonances very close to each other, this approach is unsuitable for drivers used in combination with transducers with resonant frequencies that are considerably far apart. The prior art "one dimensional" tuning methods also limit transducer performance over a wide dynamic range of drive voltages because the series and parallel resonance frequencies and admittance will vary with drive voltage, as shown in FIGS. 6, 7 and 8. FIG. 6 illustrates a three dimensional view of an actual transducer admittance magnitude as a function of frequency and drive voltage, and illustrates the obvious change in the shape of the transducer characteristics as a function of voltage and frequency. FIG. 7 illustrates the variation in admittance magnitude of the series and parallel resonance points as a function of drive voltage for the same data used to generate FIG. 6. FIG. 8 illustrates the shift in the frequency location of the series and parallel resonance points as a function of voltage for the same data used to generate FIG. 6.
Accordingly, a need continues to exist for a method tuning ultrasonic devices operating at greatly varying resonant frequencies and drive voltages.